The present invention relates to acicular magnetic iron oxide particles and a process for producing the same. More particularly, the present invention relates to acicular magnetic iron oxide particles which have a high saturation magnetization and an excellent black chromaticity, and which have a high coercive force and an excellent stability of magnetic characteristics with the passage of time in spite of a small cobalt content, and to a process for producing such acicular magnetic iron oxide particles.
With the recent development of miniaturized and light-weight magnetic recording/reading-out apparatuses, the requirement for a recording medium having a higher recording density such as a magnetic tape and a magnetic disk has been increasing more and more.
In order to improve the recording density of a magnetic recording medium, the magnetic particles as a material should be as fine as possible and should have as high a coercive force as possible. This fact is described in, for example, on pp. 185 to 187 of THE COLLECTED DATA ON MAGNETIC RECORDING MEDIA (1985), published by Synthetic Electronics Research, " - - - The recent development of the technique of magnetic tapes is remarkable - - - and high-density recording has been realized. In any of audio apparatuses, video apparatuses and floppy disk drives, the importance is placed on a miniaturization, light-weight, and good operability in addition to high-density short-wavelength signal recording technique. In order to make a magnetic film match such an apparatus, a thin film having an ultrasmooth surface of 1 to 2 .mu.m-thick has been developed by a magnetic coating technique which uses fine magnetic particles having a high coercive force. - - - A video tape of a high-grade (HG) type which uses fine magnetic particles was developed in the fall of 1982. - - - In the near future, remarkable improvement of the picture quality will be expected - - - using ultrafine magnetic iron oxide particle for a tape - - - "
In video apparatuses, since a mechanism for optically detecting the end of the running tape is generally adopted, if the light transmittance of the video tape used is high, a malfunction is apt to occur. However, if the acicular maghemite particles which are often used for a video tape due to their magnetic stability with the passage of time, are made finer, the light transmittance is inconveniently made higher.
Although an attempt to improve the light transmittance by adding a large amount of nonmagnetic filler such as carbon black has been made, the use of a nonmagnetic material such as carbon black obstructs the enhancement of high-density recording.
In order to improve the light transmittance without adding a non-magnetic filler, Co-coated acicular magnetic iron oxide particles obtained by coating with Co the surfaces of acicular magnetite particles, which are more excellent in the black chromaticity than acicular maghemite particles, have come into use.
Since acicular magnetite particles have a larger saturation magnetization than acicular maghemite particles, they are suitable for high-density recording.
However, the acicular magnetic iron oxide particles obtained by coating the surfaces of acicular magnetite particles with Co are disadvantageous in that the ferrous contained therein are gradually oxidized so that the magnetic characteristics are deteriorated with the passage of time. This tendency is more prominent as the particles are made finer.
This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 3-19127 (1991), " - - - When the Fe.sup.2+ /Fe.sup.3+ ratio in ferromagnetic iron oxide is increased, the magnetic stability is lowered, a change in the Hc of the magnetic recording medium is increased, and with this change, the signal recording property becomes unstable. Therefore, magnetic recording media using ferromagnetic iron oxide having a large Fe.sup.2+ /Fe.sup.3+ ratio are not widely used. - - - "
It is conventionally known that the larger the amount of Co coating the acicular iron oxide particles, the better the stability of the coercive force with the passage of time. Therefore, in order to improve the coercive force stability with the passage of time, namely, the magnetic stability with the passage of time, the amount of Co coating the acicular iron oxide particles is often increased. However, the cobalt supplies are unstable and the cost is often raised due to the limited producing area of cobalt or the like. It is therefore required to reduce the amount of cobalt used in the near future. As well known, if the amount of cobalt coating acicular magnetite particles is reduced, it is impossible to obtain an adequate coercive force, and the magnetic stability with the passage of time is deteriorated as described above. Accordingly, acicular magnetic iron oxide particles having an adequate coercive force and a good magnetic stability with the passage of time in spite of a small cobalt content is now in demand.
As examples of a method of producing cobalt-coated magnetic iron oxide particles having a good magnetic stability with the passage of time, the following methods are conventionally known: a method of producing a magnetite layer on the surface of acicular magnetite particles as the core particles in a cobalt coating process, producing a spinel ferrite layer composed of magnetite and a cobalt compound on the surfaces of the acicular magnetite particles by adding a cobalt salt, and further oxidizing the surfaces (Japanese Patent Application Laid-Open (KOKAI) No. 6-124827 (1994)); a method of depositing and forming a magnetite layer on the surfaces of acicular magnetite particles by the oxidation reaction of a ferrous salt, and further adding a ferrous salt and a cobalt salt so as to deposit a ferromagnetic iron oxide layer containing cobalt atoms by the oxidation reaction (Japanese Patent Publication No. 58-50005 (1983)); a method of coating acicular magnetite particles first with a ferrous compound and then coating the resultant particles with a cobalt compound (Japanese Patent Publication No. 63-23137 (1988)); a method of adding a ferrous salt to acicular magnetite particles and stirring the mixture at a temperature not higher than the boiling point for not less than 30 minutes, and adding a cobalt salt to the resultant mixture and stirring the mixture at a temperature not higher than the boiling point to coat particles with cobalt (Japanese Patent Application Laid-Open (KOKAI) Nos. 61-17426 (1986) and 61-252605 (1986)); and a method of coating the surfaces of core particles with a ferrous compound layer, forming an inner layer composed of a ferrous compound and a cobalt compound thereof, and forming as an outer layer a cobalt compound layer thereon (Japanese Patent Application Laid-Open (KOKAI) Nos. 63-295441 (1988) and 63-303817 (1988)).
Acicular magnetic iron oxide particles which have a high saturation magnetization and an excellent black chromaticity, and which have a high coercive force and an excellent magnetic stability with the passage of time in spite of a small cobalt content are now in the strongest demand. However, none of the magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-124827 (1994), Japanese Patent Publication Nos. 58-50005 (1983) and 63-23137 (1988), Japanese Patent Application Laid-Open (KOKAI) Nos. 61-17426 (1986), 61-252605 (1986), 63-295441 (1988), and 63-303817 (1988) can be said to be satisfactory.
In the magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-124827 (1994), the amount of cobalt added to the core particles is as large as 3.6 wt % based on the core particles and the coercive force is not so much improved as expected from the amount of cobalt added. In addition, a magnetite layer is formed on the surfaces of the particles in a non-oxidizing atmosphere, and electrons move from the ferrous in the adsorbed ferrous hydroxide colloid into the core particles, which damages the core particles.
The magnetic iron oxide particles described in Japanese Patent Publication No. 58-50005 (1983) are produced by forming a magnetite layer as a lower layer on the surfaces of the core particles, and simultaneously adding a ferrous salt and a cobalt salt thereto so as to form a ferromagnetic iron oxide layer containing cobalt in an oxidizing atmosphere. The ferromagnetic iron oxide layer containing cobalt formed in this manner is comparatively thick because it contains ferrous and ferric, so that the cobalt concentration becomes relatively low and the effect on the improvement of the coercive force is small.
In the magnetic iron oxide particles described in Japanese Patent Publication No. 63-23137 (1988), when the coating process is conducted at a low temperature, as shown in Examples, ferrous hydroxide colloid is difficult to adsorb onto the core particles and it floats outside of the core particles, which leads to an insufficient magnetic stability with the passage of time. The heat treatment such as autoclaving and dry heating adopted after the coating process so as to improve the magnetic stability with the passage of time furthers the diffusion of cobalt ions into the core particles, so that the effect on improvement is insufficient.
In the process for producing the magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) Nos. 61-17426 (1986) and 61-252605 (1986), an aqueous solution of a ferrous salt is added to magnetite particles and the resultant mixture is stirred at a temperature not higher than the boiling point for not less than 30 minutes. As shown in the examples, an aqueous solution of a ferrous salt is added at 70.degree. C. and the resultant mixture is stirred at 100.degree. C. When the aqueous solution is added at such a high temperature, a rapid chemical reaction is produced before a ferrous hydroxide colloid is uniformly mixed, which increases the nonuniformity in the reaction and the coercive force distribution.
In the magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) Nos. 63-295441 (1988) and 63-303817 (1988), the amount of cobalt used is as large as not less than 4 wt % based on the core particles, as shown in Examples. If the coating process is conducted at a low temperature, ferrous hydroxide colloid is difficult to adsorb onto the core particles and it floats outside of the core particles, which leads to an insufficient magnetic stability. In addition, the heat treatment such as autoclaving and dry heating adopted after the coating process so as to improve the magnetic stability does not produce a sufficient effect.
Accordingly, the technical problem of the present invention is to provide acicular magnetic iron oxide particles which have a high saturation magnetization and excellent black chromaticity, and which have a high coercive force and an excellent magnetic stability with the passage of time (magnetic stability independent to change with time) in spite of a small cobalt content.
As a result of the present inventors' studies undertaken so as to solve the above-described problems, it has been found that by adding an aqueous solution of a ferrous salt to an alkali suspension containing acicular magnetite particles having 10 to 24 wt % of ferrous at a temperature of 40.degree. to 60.degree. C. in a non-oxidizing atmosphere so as to produce a ferrous hydroxide colloid; blowing an oxygen-containing gas into the suspension so as to oxidize the colloid, thereby forming the magnetite coating (magnetite layer as a lower layer) on the surfaces of the acicular magnetite particles by epitaxial growth; further continuously conducting oxidation under predetermined conditions so as to oxidize the surface of the magnetite coating, thereby forming the maghemite at least in a part of the surface of the magnetite coating; after changing the atmosphere to a non-oxidizing atmosphere, adding an aqueous solution of a cobalt salt to the resultant suspension; and heating and stirring the obtained suspension at 60.degree. to 100.degree. C. so as to form the cobalt-containing oxide layer on the surface of the maghemite by epitaxial growth, the obtained acicular magnetic iron oxide particles comprising acicular magnetite particles coated with a magnetite coating as a lower layer formed on the surfaces of the particles by epitaxial growth, a cobalt-containing oxide layer as an upper layer in which the cobalt content is 1.5 to 2.5 wt % based on the total weight of the acicular magnetic iron oxide particles, and maghemite existing between the magnetite coating and the cobalt-containing oxide layer, have a high coercive force and an excellent magnetic stability with the passage of time. The present invention has been achieved on the basis of this finding.